Lets assume that the minimum threshold dose is 75 micrograms, almost the same amount as for a fatal dose of ricin. That means you would need 15 or so 55 gallon drums of pure LSD for our hypothetical reservoir. One would have to make sure that the reservoir was small and that it was the sole supply for the target urban area.
Have at it. Might I suggest your local university water supply where it wouldn't even be noticed.